The present invention relates to the supply of power to peripherals in general and more particularly, to a method and apparatus which enables the control of current ramp-up when a peripheral is coupled to a bus.
Computer users have found a need to couple an increasing number of peripherals to their computers. In order to accommodate this, a number of buses have been developed. One such bus is known as the Universal Serial Bus (USB). The USB is a low-to-medium speed serial bus developed by the Intel Corporation to address peripheral expansion outside the PC system box. The USB is a peripheral bus standard that permits the use of plug and play computer peripherals outside the box, reducing the need to install cards into dedicated computer slots and reconfigure the system. In personal computers equipped with USB, computer peripherals can be automatically configured as soon as they are physically attached without the need to reboot or run setup. USB also allows up to 127 devices to run simultaneously on a computer, with peripherals such as monitors and keyboards operating as additional plug-in sites, or hubs.
USB will accommodate telephones, modems, keyboards, mice, 4.times. and 6.times. CD ROM drives, joysticks, tape and floppy drives, scanners and printers. USB's 12 megabit data rate will also accommodate a whole new generation of peripherals, including MPEG-2 video-based products, data gloves and digitizers. Also, since computer-telephony integration is expected to be a big growth area for PCs, USB will provide a low-cost interface for Integrated Services Digital Network (ISDN) and digital PBXs.
Drawing its intelligence from the host PC, USB detects when devices are added and removed. The bus automatically determines the host resource, including driver software and bus bandwidth, each peripheral needs and makes those resources available without user intervention. Users with a USB-equipped PC will be able to switch out compatible peripherals as needed as easily as they would plug in a lamp.
USB transfers signals and power over a four wire cable. The signaling occurs over two wires in point-to-point segments. The cable also carries Vbus (VCC) and GND wires on each segment to deliver power to peripheral devices. Vbus is nominally +5 V at the source. Each USB segment provides a limited amount of power over the cable. The host supplies power for use by USB devices that are directly coupled to it. In addition, any USB device may have its own power supply. USB devices that rely totally on power from the cable are called bus-powered devices. In contrast, those that have an alternate source of power are called self-powered devices. A hub also supplies power for its coupled USB devices.
The connectors used with the bus are four pin connectors in which the pin assignments are as follows:
pin 1 VCC (Cable power or Vbus) PA1 pin 2-Data PA1 pin 3+Data PA1 pin 4 Ground (Cable ground)
Detailed specifications for the USB including the power supply and the connectors may be found in Universal Serial Bus Specification, Revision 1.0, Jan. 15, 1996, Copyright.COPYRGT. 1996, Compaq Computer Corporation, Digital Equipment Corporation, IBM PC Company, Intel Corporation, Microsoft Corporation, NEC, Northern Telecom.